1. Field of the Invention
The present invention relates to a wide-angle lens system comprising a first lens element, a second lens element, a third lens element, an aperture stop, and a fourth lens element, and more particularly to a super wide-angle lens system which is suitable for use in monitor or vehicle, and whose field of view is as high as 140 degrees.
2. Description of the Prior Art
The biggest problem in designing a conventional wide-angle lens system is that the problem of image distortion will occur after the object image is processed by the lens system. However, in the past, only the spherical glass lens can be manufactured and in order to compensate the negative distortion aberration (also called barrel distortion, namely, image magnification decreases with increasing distance from the optical axis, and the straight lines are curved in image), a plurality of negative and positive meniscus lens elements should be arranged at the front end of the lens system. When the field of view of the lens system is 80 degrees, it needs 8-10 lens elements, and if the field of view is more than 100 degrees, it needs 10-12 lens elements, and this will increase the length and weight of the lens system.
Due to advances in plastic optical material manufacturing and popularity of manufacturing technology of the aspheric lens element, many miniaturized and lightweight lens systems came into existence. Particularly, after using the aspheric lens element, the lens system only needs 3-4 lens elements when the field of view is 80 degrees, and it only needs 4-5 lens elements when the field of view is over 120 degrees. Obviously, lens system is developing towards miniaturization and lightweight.
Furthermore, the number of lens elements of the super wide-angle lens system disclosed in JP Patent Publication No 2003-307674, the wide-angle lens system disclosed in JP Patent Publication No 2005-227426, the wide-angle lens system disclosed in JP Patent Publication No 2006-146016, the wide-angle lens system and device for taking image as disclosed in JP Patent Publication No 2006-292988, and the optical device disclosed in JP Patent Publication No 2007-025499, has been reduced to 4-5 pieces. Therefore, there is no doubt that these lens systems are miniaturized and lightweight.
However, these lens systems still have the problem of image distortion. For a general lens system, the image distortion is usually compensated based on the protection equation: y′=f·tan ω, y is the image height, f represents focal length, and ω represents half field of view. There are also other projection methods, such as: stereographic projection y′=2f·tan(ω/2), equidistant projection y′=f·ω, equi-solid angle projection y′=2f·sin(ω/2), and orthographic projection y′=2f·sin ω. It can be seen from the above equations that when the focal length f is fixed, the image height y′ increases with the increase of the filed of view and the half field of view. When the value of y′ is too great, it is impossible to project the entire image onto the image plane, unless the peripheral image is compressed. As a result, the peripheral image will be unrecognizable. If used in a monitor, the blurred peripheral image probably makes it impossible to show the criminal face clearly. If used on a rear view monitor system of a vehicle, the blurred peripheral image probably makes the driver knock down the object or child behind the vehicle.
The current image processing technology is advanced and the distortion aberration of the lens system can be compensated by using the imaging processing technology, however, the image still has the problem of image distortion after being processed. If the distorted ad blurred peripheral image can be avoided, the wide-angle lens system will have a wider and brighter future. Therefore, based on many years of experience in lens system researching, the inventor of this invention has invented a high sharpness wide-angle lens system which can prevent extreme image distortion.